Hydraulic fracturing operations are used extensively in the petroleum industry to enhance oil and gas production. In a hydraulic fracturing operation, a fracturing fluid is injected through a wellbore into a subterranean formation at a pressure sufficient to initiate fractures to increase oil and gas production.
Frequently, particulates, called proppants, are suspended in the fracturing fluid and transported into the fractures as a slurry. Proppants include sand, ceramic particles, glass spheres, bauxite (aluminum oxide), resin coated proppants, synthetic polymeric beads, and the like. Among them, sand is by far the most commonly used proppant.
Fracturing fluids in common use include aqueous and non-aqueous ones including hydrocarbon, methanol and liquid carbon dioxide fluids. The most commonly used fracturing fluids are aqueous fluids including water, brines, water containing polymers or viscoelastic surfactants and foam fluids.
At the last stage of a fracturing treatment, fracturing fluid is flowed back to the surface and proppants are left in the fractures to prevent them from closing back after the hydraulic fracturing pressure is released. The proppant filled fractures provide high conductive channels that allow oil and/or gas to seep through to the wellbore more efficiently. The conductivity of the proppant packs formed after proppant settles in the fractures plays a dominant role in increasing oil and gas production.
However, it is not unusual for a significant amount of proppant to be carried out of the fractures and into the well bore along with the fluids being flowed back out the well. This process is known as proppant flowback. Proppant flowback is highly undesirable since it not only reduces the amount of proppants remaining in the fractures resulting in less conductive channels, but also causes significant operational difficulties. It has long plagued the petroleum industry because of its adverse effect on well productivity and equipment.
Numerous methods have been attempted in an effort to find a solution to the problem of proppant flowback. The commonly used method is the use of so-called “resin-coated proppants”. The outer surfaces of the resin-coated proppants have an adherent resin coating so that the proppant grains are bonded to each other under suitable conditions forming a permeable barrier and reducing the proppant flowback.
The substrate materials for the resin-coated proppants include sand, glass beads and organic materials such as shells or seeds. The resins used include epoxy, urea aldehyde, phenol-aldehyde, furfural alcohol and furfural. The resin-coated proppants can be either pre-cured or can be cured by an over-flush of a chemical binding agent, commonly known as activator, once the proppants are in place.
Different binding agents have been used. U.S. Pat. Nos. 3,492,147 and 3,935,339 disclose compositions and methods of coating solid particulates with different resins. The particulates to be coated include sand, nut shells, glass beads, and aluminum pellets. The resins used include urea-aldehyde resins, phenol-aldehyde resins, epoxy resins, furfuryl alcohol resins, and polyester or alkyl resins. The resins can be in pure form or mixtures containing curing agents, coupling agents or other additives. Other examples of resins and resin mixtures for proppants are described, for example, in U.S. Pat. Nos. 5,643,669; 5,916,933; 6,059,034 and 6,328,105.
However, there are significant limitations to the use of resin-coated proppants. For example, resin-coated proppants are much more expensive than normal sands, especially considering that a fracturing treatment usually employs tons of proppants in a single well. Normally, when the formation temperature is below 60° C., activators are required to make the resin-coated proppants bind together. This increases the cost.
Thus, the use of resin-coated proppants is limited by their high cost to only certain types of wells, or to use in only the final stages of a fracturing treatment, also known as the “tail-in” of proppants, where the last few tons of proppants are pumped into the fracture. For less economically viable wells, application of resin-coated proppants often becomes cost prohibitive.
During hydrocarbon production, especially from poorly consolidated formations, small particulates, typically of sand, often flow into the wellbore along with produced fluids. This is because the formation sands in poorly consolidated formations are bonded together with insufficient bond strength to withstand the forces exerted by the fluids flowing through and are readily entrained by the produced fluids flowing out of the well.
The produced sand erodes surface and subterranean equipment, and requires a removal process before the hydrocarbon can be processed. Different methods have been tried in an effort to reduce formation sand production. One approach employed is to filter the produced fluids through a gravel pack retained by a screen in the wellbore, where the particulates are trapped by the gravel pack. This technique is known as gravel packing. However, this technique is relatively time consuming and expensive. The gravel and the screen can be plugged and eroded by the sand within a relatively short period of time.
Another method that has been employed in some instances is to inject various resins into a formation to strengthen the binding of formation sands. Such an approach, however, results in uncertainty and sometimes creates undesirable results. For example, due to the uncertainty in controlling the chemical reaction, the resin may set in the well bore itself rather than in the poorly consolidated producing zone. Another problem encountered in the use of resin compositions is that the resins normally have short shelf lives. For example, it can lead to costly waste if the operation using the resin is postponed after the resin is mixed.
Thus, it is highly desirable to have a cost effective composition and a method that can control proppant flowback after fracturing treatment. It is also highly desirable to have a composition and a method of reducing formation sand production from the poorly consolidated formation.